inetpeer: remove unused list

Andi Kleen and Tim Chen reported huge contention on inetpeer
unused_peers.lock, on memcached workload on a 40 core machine, with
disabled route cache.

It appears we constantly flip peers refcnt between 0 and 1 values, and
we must insert/remove peers from unused_peers.list, holding a contended
spinlock.

Remove this list completely and perform a garbage collection on-the-fly,
at lookup time, using the expired nodes we met during the tree
traversal.

This removes a lot of code, makes locking more standard, and obsoletes
two sysctls (inet_peer_gc_mintime and inet_peer_gc_maxtime). This also
removes two pointers in inet_peer structure.

There is still a false sharing effect because refcnt is in first cache
line of object [were the links and keys used by lookups are located], we
might move it at the end of inet_peer structure to let this first cache
line mostly read by cpus.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Andi Kleen <andi@firstfloor.org>
CC: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 73 insertions, 207 deletions

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index ce616d92cc54..dafbf2c98b28 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -54,15 +54,11 @@
  *  1.  Nodes may appear in the tree only with the pool lock held.
  *  2.  Nodes may disappear from the tree only with the pool lock held
  *      AND reference count being 0.
- *  3.  Nodes appears and disappears from unused node list only under
- *      "inet_peer_unused_lock".
- *  4.  Global variable peer_total is modified under the pool lock.
- *  5.  struct inet_peer fields modification:
+ *  3.  Global variable peer_total is modified under the pool lock.
+ *  4.  struct inet_peer fields modification:
  *              avl_left, avl_right, avl_parent, avl_height: pool lock
- *              unused: unused node list lock
  *              refcnt: atomically against modifications on other CPU;
  *                 usually under some other lock to prevent node disappearing
- *              dtime: unused node list lock
  *              daddr: unchangeable
  *              ip_id_count: atomic value (no lock needed)
  */
@@ -104,19 +100,6 @@ int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries m
                                          * aggressively at this stage */
 int inet_peer_minttl __read_mostly = 120 * HZ;  /* TTL under high load: 120 sec */
 int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;      /* usual time to live: 10 min */
-int inet_peer_gc_mintime __read_mostly = 10 * HZ;
-int inet_peer_gc_maxtime __read_mostly = 120 * HZ;
-
-static struct {
-        struct list_head        list;
-        spinlock_t              lock;
-} unused_peers = {
-        .list                   = LIST_HEAD_INIT(unused_peers.list),
-        .lock                   = __SPIN_LOCK_UNLOCKED(unused_peers.lock),
-};
-
-static void peer_check_expire(unsigned long dummy);
-static DEFINE_TIMER(peer_periodic_timer, peer_check_expire, 0, 0);
 
 
 /* Called from ip_output.c:ip_init  */
@@ -142,21 +125,6 @@ void __init inet_initpeers(void)
                         0, SLAB_HWCACHE_ALIGN | SLAB_PANIC,
                         NULL);
 
-        /* All the timers, started at system startup tend
-           to synchronize. Perturb it a bit.
-         */
-        peer_periodic_timer.expires = jiffies
-                + net_random() % inet_peer_gc_maxtime
-                + inet_peer_gc_maxtime;
-        add_timer(&peer_periodic_timer);
-}
-
-/* Called with or without local BH being disabled. */
-static void unlink_from_unused(struct inet_peer *p)
-{
-        spin_lock_bh(&unused_peers.lock);
-        list_del_init(&p->unused);
-        spin_unlock_bh(&unused_peers.lock);
 }
 
 static int addr_compare(const struct inetpeer_addr *a,
@@ -203,20 +171,6 @@ static int addr_compare(const struct inetpeer_addr *a,
         u;                                                      \
 })
 
-static bool atomic_add_unless_return(atomic_t *ptr, int a, int u, int *newv)
-{
-        int cur, old = atomic_read(ptr);
-
-        while (old != u) {
-                *newv = old + a;
-                cur = atomic_cmpxchg(ptr, old, *newv);
-                if (cur == old)
-                        return true;
-                old = cur;
-        }
-        return false;
-}
-
 /*
  * Called with rcu_read_lock()
  * Because we hold no lock against a writer, its quite possible we fall
@@ -225,8 +179,7 @@ static bool atomic_add_unless_return(atomic_t *ptr, int a, int u, int *newv)
  * We exit from this function if number of links exceeds PEER_MAXDEPTH
  */
 static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
-                                    struct inet_peer_base *base,
-                                    int *newrefcnt)
+                                    struct inet_peer_base *base)
 {
         struct inet_peer *u = rcu_dereference(base->root);
         int count = 0;
@@ -235,11 +188,9 @@ static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
                 int cmp = addr_compare(daddr, &u->daddr);
                 if (cmp == 0) {
                         /* Before taking a reference, check if this entry was
-                         * deleted, unlink_from_pool() sets refcnt=-1 to make
-                         * distinction between an unused entry (refcnt=0) and
-                         * a freed one.
+                         * deleted (refcnt=-1)
                          */
-                        if (!atomic_add_unless_return(&u->refcnt, 1, -1, newrefcnt))
+                        if (!atomic_add_unless(&u->refcnt, 1, -1))
                                 u = NULL;
                         return u;
                 }
@@ -366,137 +317,96 @@ static void inetpeer_free_rcu(struct rcu_head *head)
         kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
 }
 
-/* May be called with local BH enabled. */
 static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base,
                              struct inet_peer __rcu **stack[PEER_MAXDEPTH])
 {
-        int do_free;
-
-        do_free = 0;
-
-        write_seqlock_bh(&base->lock);
-        /* Check the reference counter.  It was artificially incremented by 1
-         * in cleanup() function to prevent sudden disappearing.  If we can
-         * atomically (because of lockless readers) take this last reference,
-         * it's safe to remove the node and free it later.
-         * We use refcnt=-1 to alert lockless readers this entry is deleted.
-         */
-        if (atomic_cmpxchg(&p->refcnt, 1, -1) == 1) {
-                struct inet_peer __rcu ***stackptr, ***delp;
-                if (lookup(&p->daddr, stack, base) != p)
-                        BUG();
-                delp = stackptr - 1; /* *delp[0] == p */
-                if (p->avl_left == peer_avl_empty_rcu) {
-                        *delp[0] = p->avl_right;
-                        --stackptr;
-                } else {
-                        /* look for a node to insert instead of p */
-                        struct inet_peer *t;
-                        t = lookup_rightempty(p, base);
-                        BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
-                        **--stackptr = t->avl_left;
-                        /* t is removed, t->daddr > x->daddr for any
-                         * x in p->avl_left subtree.
-                         * Put t in the old place of p. */
-                        RCU_INIT_POINTER(*delp[0], t);
-                        t->avl_left = p->avl_left;
-                        t->avl_right = p->avl_right;
-                        t->avl_height = p->avl_height;
-                        BUG_ON(delp[1] != &p->avl_left);
-                        delp[1] = &t->avl_left; /* was &p->avl_left */
-                }
-                peer_avl_rebalance(stack, stackptr, base);
-                base->total--;
-                do_free = 1;
+        struct inet_peer __rcu ***stackptr, ***delp;
+
+        if (lookup(&p->daddr, stack, base) != p)
+                BUG();
+        delp = stackptr - 1; /* *delp[0] == p */
+        if (p->avl_left == peer_avl_empty_rcu) {
+                *delp[0] = p->avl_right;
+                --stackptr;
+        } else {
+                /* look for a node to insert instead of p */
+                struct inet_peer *t;
+                t = lookup_rightempty(p, base);
+                BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
+                **--stackptr = t->avl_left;
+                /* t is removed, t->daddr > x->daddr for any
+                 * x in p->avl_left subtree.
+                 * Put t in the old place of p. */
+                RCU_INIT_POINTER(*delp[0], t);
+                t->avl_left = p->avl_left;
+                t->avl_right = p->avl_right;
+                t->avl_height = p->avl_height;
+                BUG_ON(delp[1] != &p->avl_left);
+                delp[1] = &t->avl_left; /* was &p->avl_left */
         }
-        write_sequnlock_bh(&base->lock);
-
-        if (do_free)
-                call_rcu(&p->rcu, inetpeer_free_rcu);
-        else
-                /* The node is used again.  Decrease the reference counter
-                 * back.  The loop "cleanup -> unlink_from_unused
-                 *   -> unlink_from_pool -> putpeer -> link_to_unused
-                 *   -> cleanup (for the same node)"
-                 * doesn't really exist because the entry will have a
-                 * recent deletion time and will not be cleaned again soon.
-                 */
-                inet_putpeer(p);
+        peer_avl_rebalance(stack, stackptr, base);
+        base->total--;
+        call_rcu(&p->rcu, inetpeer_free_rcu);
 }
 
 static struct inet_peer_base *family_to_base(int family)
 {
-        return (family == AF_INET ? &v4_peers : &v6_peers);
+        return family == AF_INET ? &v4_peers : &v6_peers;
 }
 
-static struct inet_peer_base *peer_to_base(struct inet_peer *p)
+/* perform garbage collect on all items stacked during a lookup */
+static int inet_peer_gc(struct inet_peer_base *base,
+                        struct inet_peer __rcu **stack[PEER_MAXDEPTH],
+                        struct inet_peer __rcu ***stackptr)
 {
-        return family_to_base(p->daddr.family);
-}
-
-/* May be called with local BH enabled. */
-static int cleanup_once(unsigned long ttl, struct inet_peer __rcu **stack[PEER_MAXDEPTH])
-{
-        struct inet_peer *p = NULL;
-
-        /* Remove the first entry from the list of unused nodes. */
-        spin_lock_bh(&unused_peers.lock);
-        if (!list_empty(&unused_peers.list)) {
-                __u32 delta;
+        struct inet_peer *p, *gchead = NULL;
+        __u32 delta, ttl;
+        int cnt = 0;
 
-                p = list_first_entry(&unused_peers.list, struct inet_peer, unused);
+        if (base->total >= inet_peer_threshold)
+                ttl = 0; /* be aggressive */
+        else
+                ttl = inet_peer_maxttl
+                                - (inet_peer_maxttl - inet_peer_minttl) / HZ *
+                                        base->total / inet_peer_threshold * HZ;
+        stackptr--; /* last stack slot is peer_avl_empty */
+        while (stackptr > stack) {
+                stackptr--;
+                p = rcu_deref_locked(**stackptr, base);
                 delta = (__u32)jiffies - p->dtime;
-
-                if (delta < ttl) {
-                        /* Do not prune fresh entries. */
-                        spin_unlock_bh(&unused_peers.lock);
-                        return -1;
+                if (atomic_read(&p->refcnt) == 0 && delta >= ttl &&
+                    atomic_cmpxchg(&p->refcnt, 0, -1) == 0) {
+                        p->gc_next = gchead;
+                        gchead = p;
                 }
-
-                list_del_init(&p->unused);
-
-                /* Grab an extra reference to prevent node disappearing
-                 * before unlink_from_pool() call. */
-                atomic_inc(&p->refcnt);
         }
-        spin_unlock_bh(&unused_peers.lock);
-
-        if (p == NULL)
-                /* It means that the total number of USED entries has
-                 * grown over inet_peer_threshold.  It shouldn't really
-                 * happen because of entry limits in route cache. */
-                return -1;
-
-        unlink_from_pool(p, peer_to_base(p), stack);
-        return 0;
+        while ((p = gchead) != NULL) {
+                gchead = p->gc_next;
+                cnt++;
+                unlink_from_pool(p, base, stack);
+        }
+        return cnt;
 }
 
-/* Called with or without local BH being disabled. */
 struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
 {
         struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
         struct inet_peer_base *base = family_to_base(daddr->family);
         struct inet_peer *p;
         unsigned int sequence;
-        int invalidated, newrefcnt = 0;
+        int invalidated, gccnt = 0;
 
-        /* Look up for the address quickly, lockless.
+        /* Attempt a lockless lookup first.
          * Because of a concurrent writer, we might not find an existing entry.
          */
         rcu_read_lock();
         sequence = read_seqbegin(&base->lock);
-        p = lookup_rcu(daddr, base, &newrefcnt);
+        p = lookup_rcu(daddr, base);
         invalidated = read_seqretry(&base->lock, sequence);
         rcu_read_unlock();
 
-        if (p) {
-found:          /* The existing node has been found.
-                 * Remove the entry from unused list if it was there.
-                 */
-                if (newrefcnt == 1)
-                        unlink_from_unused(p);
+        if (p)
                 return p;
-        }
 
         /* If no writer did a change during our lookup, we can return early. */
         if (!create && !invalidated)
@@ -506,11 +416,17 @@ found:		/* The existing node has been found.
          * At least, nodes should be hot in our cache.
          */
         write_seqlock_bh(&base->lock);
+relookup:
         p = lookup(daddr, stack, base);
         if (p != peer_avl_empty) {
-                newrefcnt = atomic_inc_return(&p->refcnt);
+                atomic_inc(&p->refcnt);
                 write_sequnlock_bh(&base->lock);
-                goto found;
+                return p;
+        }
+        if (!gccnt) {
+                gccnt = inet_peer_gc(base, stack, stackptr);
+                if (gccnt && create)
+                        goto relookup;
         }
         p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
         if (p) {
@@ -525,7 +441,6 @@ found:		/* The existing node has been found.
                 p->pmtu_expires = 0;
                 p->pmtu_orig = 0;
                 memset(&p->redirect_learned, 0, sizeof(p->redirect_learned));
-                INIT_LIST_HEAD(&p->unused);
 
 
                 /* Link the node. */
@@ -534,63 +449,14 @@ found:		/* The existing node has been found.
         }
         write_sequnlock_bh(&base->lock);
 
-        if (base->total >= inet_peer_threshold)
-                /* Remove one less-recently-used entry. */
-                cleanup_once(0, stack);
-
         return p;
 }
-
-static int compute_total(void)
-{
-        return v4_peers.total + v6_peers.total;
-}
 EXPORT_SYMBOL_GPL(inet_getpeer);
 
-/* Called with local BH disabled. */
-static void peer_check_expire(unsigned long dummy)
-{
-        unsigned long now = jiffies;
-        int ttl, total;
-        struct inet_peer __rcu **stack[PEER_MAXDEPTH];
-
-        total = compute_total();
-        if (total >= inet_peer_threshold)
-                ttl = inet_peer_minttl;
-        else
-                ttl = inet_peer_maxttl
-                                - (inet_peer_maxttl - inet_peer_minttl) / HZ *
-                                        total / inet_peer_threshold * HZ;
-        while (!cleanup_once(ttl, stack)) {
-                if (jiffies != now)
-                        break;
-        }
-
-        /* Trigger the timer after inet_peer_gc_mintime .. inet_peer_gc_maxtime
-         * interval depending on the total number of entries (more entries,
-         * less interval). */
-        total = compute_total();
-        if (total >= inet_peer_threshold)
-                peer_periodic_timer.expires = jiffies + inet_peer_gc_mintime;
-        else
-                peer_periodic_timer.expires = jiffies
-                        + inet_peer_gc_maxtime
-                        - (inet_peer_gc_maxtime - inet_peer_gc_mintime) / HZ *
-                                total / inet_peer_threshold * HZ;
-        add_timer(&peer_periodic_timer);
-}
-
 void inet_putpeer(struct inet_peer *p)
 {
-        local_bh_disable();
-
-        if (atomic_dec_and_lock(&p->refcnt, &unused_peers.lock)) {
-                list_add_tail(&p->unused, &unused_peers.list);
-                p->dtime = (__u32)jiffies;
-                spin_unlock(&unused_peers.lock);
-        }
-
-        local_bh_enable();
+        p->dtime = (__u32)jiffies;
+        atomic_dec(&p->refcnt);
 }
 EXPORT_SYMBOL_GPL(inet_putpeer);